Examination of Firearms Review: 2007 to 2010 - Interpol

same paper towel is used on an un-treated surface, new fingermarks can readily be
detected. The lifting of the marks was however impossible and all marks have to be
photographed in situ. Further research is underway to elucidate the chemical and
physical underlying mechanisms responsible for this phenomenon.
Two commercially available white powder suspensions (WPS) were tested (i.e.,
Wetwop and Wet Powder) in comparison with a “home-made” titanium dioxide-based
formula (242, 243). Tests were performed with depleted series of one-day-old
fingermarks left on some non-porous surfaces (e.g., various coloured plastic bags
and black plastic-based car cowlings), before being immersed for six hours in water,
dried (from two to eight days), and eventually processed. The results showed that all
three WPS formulations led to similar results, with a slight preference for Wetwop for
the least amount of background staining left after rinsing. On the same basis, the
authors further compared the Wetwop SPR formulation with vacuum metal deposition
(VMD) to detect fingermarks on wetted non-porous surfaces (246). The same kinds
of substrates were used, and the fingermark deposition procedure was kept
unchanged (except that drying times were of two to 28 days). Fingermarks were cut
in halves and processed separately with the two techniques. Both techniques gave a
large percentage of potentially identifiable marks across all substrates and ages (at
the exception of the car cowling for which Wetwop outperformed VMD). Given that
VMD requires extremely costly equipment and is a labor-intensive technique
compared to powder suspensions, Wetwop is recommended to be applied when
dealing with wetted non-porous surfaces. For those possessing a VMD machine,
Wetwop could still be applied subsequently, in a sequence.
Since several researches observed variations in the effectiveness of commercially
available powder suspensions (in terms of ridge quality and background staining),
Jones et al. compared four WPS formulas (i.e., TiO2 grade RG-15 - StanChem, Wet
Powder white – Kjell Carlsson Innovation, Wetwop white – Armor Forensics, and
Adhesive side Powder light - Sirchie) (244). The WPS formulas were applied on
black insulating tape and the obtained results were analytically characterized using
electron microscopy and X-ray photoelectron spectroscopy. The particle size
distribution was quite similar for all brands (diameter range: 200-500 nm) and did not
seem to play a role in the difference of effectiveness. On the other hand, the coating
of the particles varies between the WPS formulations (in terms of morphology and
chemical composition), and is more likely to explain the difference of performance
observed between the different brands (245). In the same context, Jones et al. used
atomic force microscopy and scanning electron microscopy to investigate the
smoothness of surfaces classified as non-porous (i.e., formica, polyethylene, and
unplasticised polyvinyl chloride) (248). They subsequently tried to correlate these
analytical measures with the effectiveness of iron oxide powder suspension to detect
fingermarks on those substrates (18-hour-old sebum-rich marks were used). They
showed that both average roughness and topographical feature shape are important
factors to consider for the processing of latent fingermarks.
A detrimental effect of cyanoacrylate fuming has been observed if the items
(latex/nitrile gloves or duct tapes) have to be subsequently processed by black
powder suspension (233), as described in the “Cyanoacrylate fuming” section.
256